Field of Invention
The present invention relates generally to monitoring impairments in a Hybrid Fiber-Coax (HFC) network, and more particularly to apparatus and methods for detecting and locating linear impairments such as, but not limited to, impedance mismatches causing micro-reflections in a coaxial cable plant of an HFC network.
Background Art
Micro-reflections are one of the most critical impairments in an HFC network, especially in the return path where signal losses in the cable are minimal and reflected signals from even remote impedance mismatches can impact upstream service signals from DOCSIS cable modems (CMs). Detection and location of mismatches can be a challenging problem in an HFC network carrying signal traffic or service signals (“live HFC network”). Traditional methods of detection and location have involved Time-Domain Reflectometry (TDR). In general, TDR involves the injection of high-powered probe signals or pulses into the network and the measurement of time delay, level and polarity of the reflected probe signal/pulse from mismatches in the network. Employment of TDR in a live HFC network is challenging because of the number of service signals present and the fact that the service signals can be disrupted by the probe signal and can interfere with the detection of reflected probe signals (e.g., a low-level reflected probe signal can be masked by strong service signals). Thus, heretofore, TDR has not been very practicable for testing in a live HFC network.
A technique for detecting mismatches in the return path of a live HFC network without injecting a probe signal has been developed by CableLabs® as part of the Proactive Network Maintenance (PNM) initiative. It is described in a published document entitled, DOCSIS® Best Practices and Guidelines, Proactive Network Maintenance Using Pre-Equalization, CM-GL-PNMP-V02-110623, by Cable Television Laboratories, Inc., Louisville, Colo., Jun. 23, 2011 (http://www.cablelabs.com/wp-content/uploads/2014/05/CM-GL-PNMP-V02-110623.pdf). The PNM technique is based on using pre-equalization coefficients from CMs operating in the HFC network. This technique can be classified as virtual TDR, because the upstream QAM service signals serve as the probe signals. Briefly, this technique can detect a micro-reflection of an upstream QAM signal (transmitted from a CM to a Cable Modem Termination System) and measure the time delay of the micro-reflection (relative to the initial signal) by analysis of pre-equalization coefficients. Then, by using electronic maps, a probable point in the network where the micro-reflection occurred (impedance mismatch) is located. The declared accuracy of the PNM technique is about +/−50 feet. This is not as accurate as traditional TDR using a wideband probe signal. The accuracy of the PNM technique is limited by the bandwidth of one upstream QAM channel (6.4 MHz in the U.S.) and by the number of pre-equalization coefficients. In addition, the PNM technique usually identifies only a branch in the coaxial cable portion of the network where the mismatch is located. Despite these limitations, PNM technology is widely used in cable TV HFC networks today.
Another technique for locating mismatches in a live HFC network uses downstream signals of the HFC network. This technique is disclosed in U.S. Pat. No. 6,385,237 to Tsui et al. This patent indicates that it discloses a method and system for non-invasive testing of a digital cable network. No probe signal is injected into the network. Amplitude measurements are made over a wide band of frequencies of the downstream spectrum and the resulting spectrum of measurements are then converted to the time domain. An adaptive filter then seeks to match its output to the time domain representation, thereby characterizing the communication channel (i.e., the cable). It is stated that impedance mismatches may be located using this technique and that the accuracy of location may be to within 0.7 meters (Col. 10, lines 6-11).
CableLabs® has also proposed to use the full downstream signal to detect and locate impedance mismatches in a live HFC network. See, e.g., presentation entitled, Distortion Testing in Home Terminals, CableLabs® SCTE Conference Demo, Oct. 21-24, 2013, by Thomas Williams and Alberto Campos. In this technique, the full-spectrum downstream signal is captured by a modern CM (e.g., containing a new Broadcom® chipset) and then the autocorrelation function of the signal (which may contain a reflected signal) is calculated. Again, no probe signal is injected into the network. If a reflection exists, it would appear as a spike (in addition to the main detection peak) in the autocorrelation function. Thus, the time-distance between the main peak and the reflection spike can be measured. This technique allows one to measure the time delay of a reflection more accurately (thus pinpointing the location more accurately) than the PNM upstream technique identified above. However, this downstream technique has some drawbacks. First, to provide good detection sensitivity, the accumulation time for estimating the autocorrelation function should be relatively long (e.g., tens of milliseconds), while the duration of the captured full downstream signal is limited to only tens of microseconds. The capture duration is limited to achieve a reasonable resolution bandwidth for spectral analysis (e.g., 30 kHz) and a reasonable data size (e.g., 30 Kbytes). Therefore, to achieve an adequate accumulation time it may be necessary to carry out a large number of captures, which could take time and impact CM signal traffic. Another limitation is that continuous wave (CW) or narrowband downstream signals, such as analog video carriers, AGC pilots, and OFDM pilot subcarriers, may be included in the autocorrelation, which could contribute flat or spread components to the function. These flat or spread components could mask actual reflection spikes and reduce sensitivity and time resolution (distance accuracy).
Another known technique, somewhat similar to CableLabs' full downstream capture technique, is disclosed in U.S. Pat. No. 7,271,596 to Furse et al. In Furse et al., reflections from impedance mismatches are also detected by obtaining an autocorrelation function of a full-spectrum service signal (an “operational signal”). However, in a modern HFC network, the capturing and processing of the full-spectrum operational signal and estimating its autocorrelation function is not a trivial task and has certain drawbacks as discussed above. A downstream signal in an HFC network has a very wide bandwidth (e.g., 1 GHz with a probable increase to 1.7 GHz under DOCSIS 3.1). Therefore, sampling and digitally processing this signal requires expensive and high power-consuming chipsets. Also, TDR is generally used in a section of a coaxial plant that contains only passive network devices (“passive section”), e.g., between adjacent trunk amplifiers. Downstream signals that pass through amplifiers may already contain reflected signals which could, in an autocorrelation function, mask reflections originating in the passive section under test.
The use of spread spectrum modulation of a TDR probe signal is also a known technique for testing a communications network carrying service signals. See, for example, U.S. Pat. No. 7,069,163 to Gunther, where a digital data signal is transmitted in an aircraft communication cable and a probe signal is created by spread spectrum modulating the digital data signal. Then, the spread spectrum probe signal is injected into the cable while the cable is carrying the original unaltered digital data signal. A reflected probe signal is received and cross-correlated with the original probe signal to obtain a correlation peak, which represents a time delay (roundtrip propagation time) associated with a mismatch. A matched filter implementation is also described in connection with FIG. 7. Direct Sequence Spread Spectrum, Frequency Hopped Spread Spectrum, and Zero Coding Spread Spectrum are listed as types of spread spectrum that could be used. This technique works well in the case where the digital data signal is fully random and its autocorrelation function approximates the Dirac delta function. But, the spectrum of service signals in the return path of an HFC cable television network contains a group of adjacent QAM signals separated by guard bands and also contains narrowband signals, such as set top box FSK signals. So, despite the random nature of each QAM signal, the autocorrelation function of a full return spectrum (to achieve optimal time resolution) will contain a number of high-level sidelobes which could mask the reflected signal. Another problem, as noted above, is potential interference by high-level service signals with the reflected signal. A further problem is that the return service signals may already be pre-corrected from PNM pre-equalization at the CM; thus, the autocorrelation function of these signals may contain extra peaks (caused by pre-equalization processing) and such peaks will produce false alarms (false detection) of impedance mismatches. Thus, the technique may not satisfy the need for an accurate, effective and reliable TDR test meter for the return path of an HFC cable television network.
As already suggested, the preferred approach to accurately pinpointing locations of micro-reflections in an HFC cable television network is to use a TDR meter in the field. Therefore, efforts have been made to develop handheld, battery-powered TDR test meters capable of being coupled to the network at an appropriate and convenient test point (e.g., amplifier test port). The meter is used to locate an impedance mismatch and, once a repair has been made, to confirm that the mismatch has been fixed. In the field, an opportunity may not be available to poll data from nearby CMs to determine the presence of micro-reflection distortion. Also, implementing a technique that captures the whole downstream signal in a handheld meter for analysis may not be cost effective, because high resolution analog-to-digital converter chips with 2 GHz plus sampling rates are needed to capture the downstream signal. The chips are still very expensive and have high power consumption (e.g., a few Watts). Further, to undertake the necessary signal processing of a full downstream spectrum, a powerful, high cost, high power-consuming, computer or processor would be required.
In view of the above discussion, it is apparent that a need exists for a non-invasive, low-cost, low power-consuming, handheld TDR meter for detecting micro-reflections and locating their cause in a live HFC cable television network.